We need the pinned messages to be size customizable.
Resize message icons- want rows of four not three
It seems your question is about an existing app, not one you develop. And you would like to have a change (in messages app ?).
If so, the question or request should better be posted in Feedback Assistant https://feedbackassistant.apple.com
There's no supported way for you to change the pinned icon size with the APIs currently available. If you'd like us to consider adding the necessary functionality, please file an enhancement request using Feedback Assistant. Once you file the request, please post the FB number here. If you're not familiar with how to file enhancement requests, take a look at Bug Reporting: How and Why?